This invention relates to vehicle locating systems and more specifically to vehicle locating systems useful in an air traffic control environment which is based upon the use of satellite and ground station components to provide location accuracies superior to those based entirely upon either satellite or ground station components alone.
As air traffic has increased, it has become desirable to reduce the separation between aircraft and thereby increase the available capacity of the nation's controlled air space. The air traffic control system in the United States has as its primary performance criterion the safety of air traffic. Under such circumstances and criterion, uncertainty of the location of aircraft directly translates into large separation requirements between aircraft. Furthermore, present systems which are primarily radar based require the use of transponders aboard the aircraft which must be set by the pilot to a predetermined code in order to identify the aircraft. If the aircraft is equipped with a mode C transponder, altitude may be transmitted on the transponder's signal to an interrogating radar, but such an altitude signal is dependent upon the availability on the aircraft of an altitude encoder in addition to the radar transponder. Radar coverage does not exist throughout the United States but is generally concentrated in areas of high population or frequent on-route flights. As a result, the air traffic control system is severely limited in it's capacity in areas where radar coverage is not available and only the pilot's reporting of position may be relied upon to locate the aircraft. As a result, large separations between aircraft must be provided.
In the present air traffic control system, location of aircraft is provided to a human air traffic controller who provides separation and conflict alerting on the basis of the enhanced radar display before him. While certain computerized conflict alert systems are available, such systems are not very useful in highly congested areas, since the location accuracies of the radar system and the performance envelopes of the aircraft cause many false alarms. Thus, poor location accuracy reduces the air traffic system capacity and the ability to perform computed conflict alert in congested air space.
Satellite based vehicle location systems, such as the Geostar System described in U.S. Pat. No. 4,359,733, dated Nov. 16, 1982 have been proposed to address the accuracy and capacity problems of the existing air traffic control system and to automate the reporting of potential conflicts to pilots based upon information derived from computer determination of aircraft location for aircraft equipped with the appropriate electronic equipment. However, such systems have generally required relatively expensive airborne equipment and complex and expensive satellite systems in order to provide the services required. Furthermore, while they are capable of providing improved accuracies compared to radar based systems and coverage of essentially all of the continental United States by the use of a few satellites, such systems suffer from Geometric Dilution of Precision (GDOP) due to the small crossing angles of the hyperbola of lines of position derived from the vehicle transmissions. For geostationary satellite repeater systems, GDOP factors greater than 100 are typical. A system constructed with ground based repeaters would suffer from a similar GDOP problem in the elevation plane, since the crossing angles of the lines of position for such a system are also small. These magnitudes of GDOP require larger than desired spacing between aircraft.
From the above, it is obvious that neither the systems based entirely on a satellite or satellites nor the systems based entirely on a ground station or stations satisfies the positional accuracy and cost requirements of a next generation air traffic control system. Such a system should minimize the cost and complexity of airborne and spaceborne components and still require a relatively small number of ground stations while providing continental United States coverage.